The present invention relates to electrophotographic carrier particles, and particularly to carrier particles for electrophotographic developers which provide improved solid area development.
Electrophotographic processes and apparatus employ the use of toners, which typically comprise a resin and a colorant, along with other desirable additives like charge control agents. In general, a desired image is formed on an organic photoconductor (OPC) coated medium such as a drum or belt in the form of a charged pattern representing the image. Toner is electrically attracted to the charge on the drum and adheres to the drum in an imagewise manner. Lastly, the toner image is transferred from the OPC medium to an image-receiving substrate (typically paper) and fused, resulting in permanent image formation on the substrate.
In magnetic brush development systems, charge is imparted to the toner triboelectrically by mixing toner particles with carrier particles, typically resin-coated steel particles about 20 to 200 .mu.m in diameter. The toner particles adhere to the oppositely-charged carrier particles and are conveyed from a hopper to the magnetic brush roller system. On the roller, chains of the toner-laden carrier particles form, and as the chains are conveyed on the roller into the gap between the roller and the OPC medium, the charged toner particles are attracted to and deposited on the oppositely-charged latent image areas of the OPC medium. The carrier particles are collected and recycled for remixing with toner.
Since the carrier is a recyclable component of the developer, it is desirable to make the carrier last as long as possible, to minimize cost of usage. After a period of mixing with toner, toner particles tend to irreversibly adhere to the carrier, rendering triboelectric charging ineffective and necessitating replacement of the carrier. This is a problem sometimes referred to as "toner scum", and is noted with fluoropolymer coating materials such as polytetrafluoroethylene (PTFE). However, these materials are triboelectrically desirable for use in making carrier; fluoropolymers typically have a low surface free energy due to the presence of carbon-to-fluorine bonds and, as such, make ideal materials for carrier coating. Toner filming or scum may be suppressed by incorporating certain silicones and copolymers of tetrafluoroethylene (TFE), p-vinylidene fluoride and the like. However, the lack of solubility in common organic solvents and lack of adhesion to ferromagnetic substrate materials effectively precludes their use with normal fluidic coating equipment and processes. The lack of adhesion problem has been addressed by the provision of another agent such as a heat-curable epoxy system to adhere the PTFE to the substrate, but this solution is less than desirable, since the presence of the epoxy alters the characteristics of the end-product carrier material.
Another problem in the art relates to solid area development and the control thereof. In carrier/toner systems, uniform application of toner, i.e., "toning," across relatively large area of the document is desired. This is commonly called "solid area fill." During formation of a latent image on the photoconductor surface, an electric field is formed of the size and shape of the optically projected image (i.e., "imagewise".) Electrostatic field lines of force tend to migrate to the edges of the latent image field, and toner, during development, follows these lines of force. If the shape of the field is not corrected, most toner will be deposited along the edges of the latent image field, resulting in little or no development of the interior of the image, a condition known as "hollow character defect" or "edging."
A generally applied solution to correct this defect is to move a conductive bar or the like into the field, whose force lines project into space. This has the effect of making the field lines project perpendicularly to the photoconductor surface and to space themselves evenly across the large solid image field. This effect is commonly known as the "development electrode effect;" the conductive material is termed the "development electrode."
Ferromagnetic carriers used in magnetic brush development take the place of solid development electrodes; if they are sufficiently conductive, the carrier renders excellent solid area fill to large image areas. The conductivity of the carrier particle determines the strength of the development electrode effect.
Examples of carrier core materials used in the prior art range from extremely resistive flint glass (which is only able to develop solid areas not larger than ordinary type fonts); to powdered iron and steel, which develops excellent solid area fill, but is highly susceptible to either rusting in high moisture environments, or the formation of "scale" which interferes with carrier coating adhesion. These core materials must be passivated and cleaned, either chemically or by surface oxidation.
Synthetic ferrite core materials are not rendered useless by moisture, since they are formed from metal oxides. They are more resistive than iron and more conductive than glass beads. To improve their solid area image development, however, it is usually necessary to incorporate electroconductive particles in the coating to enhance the development electrode effect.
Improved solid area fill arises from using greater amounts of electroconductive particles, but in prior art systems, large amounts (e.g., 10-12% by weight of coating) of carbon are used, resulting in the coating becoming so conductive as to preclude sufficient charging of toner, as evidenced by higher "background" and excessively dirty machines, as well as low yield (number of copies/cartridge.) Accordingly, improved development systems addressing these issues have been desired.